1. Field of the Invention
The present invention relates to a magnetic recording medium for heat-assisted recording devices, and to a method of manufacturing the magnetic recording medium.
2. Description of the Related Art
Perpendicular magnetic recording schemes are used as a technique for achieving magnetic recording media of higher density. Media (perpendicular magnetic recording media) in which magnetic recording is performed according to this scheme comprise a non-magnetic substrate that is formed out of a non-magnetic material, and a magnetic recording layer that is formed out of a magnetic material. The medium may further comprise, for instance, a backing formed of a soft magnetic material and that has the function of concentrating, in the magnetic recording layer, the magnetic flux that is generated by a magnetic head; a ground layer that orients the magnetic recording layer in a target direction; and a protective layer that protects the surface of the magnetic recording layer.
Magnetic recording layers for perpendicular magnetic recording media that have been proposed include, for instance, a magnetic recording layer (granular magnetic layer) having a granular structure resulting from adding a non-magnetic material such as SiO2, TiO2 or the like to an alloy material such as CoCrPt, CoCrTa or the like (Japanese Patent Application Publication No. 2001-291230). In a CoCrPt—SiO2 granular magnetic layer, for instance, a SiO2 non-magnetic material segregates so as to surround the periphery of CoCrPt magnetic crystal grains, so that the individual magnetic crystal grains of CoCrPt are magnetically isolated by the SiO2 non-magnetic material.
In recent years, magnetization reversal units have become smaller as a result of a reduction in the size of magnetic crystal grains in granular magnetic layers. Recording bits, as signal units that are recorded by a magnetic head, have also become smaller. This entails a need for higher recording densities in perpendicular magnetic recording media. However, the thermal stability of recording magnetization drops as the size of the magnetic crystal grains decreases. Accordingly, it has been proposed to form magnetic crystal grains, in a granular magnetic layer, out of materials having higher magnetocrystalline anisotropy, in order to compensate for the drop in thermal stability that accompanies a reduction in the size of the magnetic crystal grains.
Herein, L10 ordered alloys are materials having the required high magneto-crystalline anisotropy. Various methods have been proposed for manufacturing thin films of L10 ordered alloys (Japanese Patent No. 3318204). Ordinarily, aluminum or glass-made non-magnetic substrates are used in magnetic recording media from the viewpoint of strength, impact resistance and so forth. A ground layer is important in a case where a L10 ordered alloy layer is formed on the surface of such a non-magnetic substrate. That is because, in order to impart high magnetocrystalline anisotropy to the magnetic crystal grains, the crystals of the L10 ordered alloy must take on a (001) orientation (the [001] axis of the crystals must be perpendicular to the substrate surface). Accordingly, MgO or SrTiO3 are ordinarily used as underlayers, which have appropriate lattice misfit properties (high lattice matching) for L10 ordered alloys.
The thickness of the magnetic recording layer is uniform in the in-plane direction of the medium. Therefore, a smaller magnetization reversal unit translates into a smaller cross-sectional area keeping a constant height of the magnetization reversal unit. As a result, the demagnetizing field that acts on the magnetization reversal unit itself becomes smaller, and the switching field becomes greater. In terms of the shape of the magnetization reversal unit, a greater write magnetic field is required in order to increase the recording density required.
For the problem of writing ability, a recording scheme has been proposed, known as heat-assisted recording, which focuses on a combination of a magnetic recording medium and a head. Heat-assisted recording exploits the temperature dependence of the magnetic anisotropy constant Ku of magnetic materials, i.e., the characteristic whereby Ku becomes smaller at higher temperatures. In heat-assisted recording, the magnetic recording medium is heated to temporarily lower thereby the Ku of a magnetic recording layer and reduce the switching field as a result. Writing is carried out during that temporary interval. Once the temperature reverts (drops) to its earlier value, Ku takes on its original high value. A recording signal can be held stably as a result. In the manufacture of magnetic recording media that are appropriate for heat-assisted recording, the design of the magnetic recording layer mandates that temperature characteristics also be taken into consideration, in addition to conventional guidelines.
The transition width of the recording bit is determined by the size of the magnetization reversal unit of the magnetic recording medium and by the magnetic field gradient and temperature gradient of the head. The transition width of recording bits must be reduced in order to enhance recording density. In particular, it is important that the temperature gradient of a heating spot generated by a laser beam should be steep. Regarding this approach a method has been proposed where, for instance, a heat dissipation layer is provided within a magnetic recording medium (Japanese Patent Application Publication No. 2010-182386). The heat dissipation layer must have a thickness according to the heating spot, but, as described above, the heat dissipation effect of the heat dissipation layer is greater the thicker the layer is. A thicker heat dissipation layer, however, entails larger irregularities on the magnetic recording medium surface. Large irregularities in a layer that underlies the magnetic recording layer exert an influence on the fine structure of the overlying magnetic recording layer, in particular on the crystal orientability and size control of magnetic crystal grains, in that crystal orientability becomes poorer, and particle size variation becomes greater. An excessively large heat dissipation layer constitutes therefore an obstacle for achieving a high-density magnetic recording medium.
To deal with this problem, the abovementioned magnetic recording medium has a layer structure such that a soft magnetic underlayer is sandwiched between two heat dissipation layers. Preferably, the layer configuration is such that the heat dissipation layer and the soft magnetic underlayer have each a plurality of layers, and heat dissipation layers and soft magnetic underlayers are stacked alternately with each other. Methods have been disclosed thus wherein the heat dissipation effect is enhanced, the irregularities at the magnetic recording medium surface are reduced, and a higher density is achieved. Further, a magnetic recording medium has been proposed, as a medium compatible with heat-assisted recording, that combines high-density writing with control of the temperature characteristic (Japanese Patent Application Publication No. 2009-93780).
As described above, thickening of the heat dissipation layer with a view to enhancing the heat dissipation effect constitutes an obstacle for achieving a magnetic recording medium having higher density. In particular, excessively large irregularities at the magnetic recording medium surface result in unstable flying of the magnetic head, causing fluctuations of gap length during signal writing and reading, and resulting in lowered SNR (signal to noise ratio), among other drawbacks. If a configuration is resorted to wherein a soft magnetic underlayer is sandwiched between two heat dissipation layers, it becomes necessary to provide the number of chambers according to the number of the plurality of heat dissipation layers, during production in a vacuum batch film-formation line, from the bottom layer to the top layer. Thus, equipment costs undesirably increase in proportion to the number of chambers.